Description of the Prior Art
In our current industrial society, fossil fuels such as petroleum, coal, natural gas, etc. have been consumed in huge amounts, and this consumption has discharged large quantities of NO.sub.x, SO.sub.x and CO.sub.2 and the like. This discharge has caused a great number of problems, such as environmental pollution in the form of acid rain, the greenhouse effect in warming the earth, and the like. Further more, it is said that deposits of fossil fuels are limited and will be used up in the near future, causing important social problems.
Accordingly, new clean energy sources free of environmental pollution have been sought as a substitute for fossil fuels have been desired worldwide. Attention has currently focused on alcohol and methane gas as energy sources for the next generation to replace petroleum. However, alcohol and methane gas still present problems, as they produce large amounts of CO.sub.2 upon combustion.
Therefore, attention has been directed to hydrogen gas. The exothermic energy per unit weight in combustion for hydrogen gas is three times that of petroleum, and its only by-product is water. It is thus expected that hydrogen gas would be an ideal clean energy source for the next generation.
However, hydrogen gas is currently produced on an industrial scale by means of electrolysis of water, high pressure thermal decomposition of natural gas, and the like. These methods require fossil fuels as energy sources to obtain the hydrogen gas. Unless the problems of energy sources for these methods of obtaining hydrogen gas are solved, the various problems described above, including environmental pollution, are not fundamentally solved.
Currently, attention is paid to microorganisms, and several studies on production of hydrogen gas by microorganisms have been made. Certainly if a method for producing hydrogen gas by microorganisms were established, the method would involve advantages that the system is easily constructed and energy consumption is extremely small, since the reaction is conducted at ambient temperature under ambient pressure. Furthermore, the raw material for producing hydrogen gas is reproducible biomass, and this biomass is obtained by photosynthesis using solar energy. Furthermore, the production of hydrogen gas by microorganisms has another advantage in that environmental pollution is solved due to efficient treatment of waste water, since it is generally possible to use as a raw material an organic substance present in wastes or in waste water.
As described above, several studies were made on the production of hydrogen gas by microorganism and, as a result, some microorganisms capable of producing hydrogen gas were found. These known microorganisms capable of producing hydrogen gas are roughly classified into photosynthetic microorganisms and non-photosynthetic bacteria. The former group includes Rhodobacter sphaeroides, which is a photosynthetic bacterium, and Oscillatoria sp. Miami BG7, which is a blue-green algae. The latter includes Azobacter chroococuum and Klebsiella pneumoniae, which are nitrogen fixing bacteria, Escherichia coli and Enterobacter aerogenes, which are facultative anaerobic bacteria, and Clostridium butyricum, which is an anaerobic bacterium.
Indeed, the production of hydrogen gas by microorganisms has many advantages as a method for preparing hydrogen gas. However, in practice, this is not practical on an industrial scale. No microorganisms having a suitably high productivity of hydrogen gas that would make it useful on an industrial scale has been found in the studies made so far. As described above, the technique for producing hydrogen gas by microorganisms has not yet reached the industrial level.
None of the currently known microorganisms exhibits good production of hydrogen gas. Photosynthetic microorganisms also require fermenters having a large surface area and large quantities of water for producing hydrogen gas, since solar energy is used.
On the other hand, it is possible to produce hydrogen gas by non-photosynthetic bacteria in a small fermenter. Such a fermenter may be installed underground and, therefore, there is a wide selection of locations for the fermenter. Because non-photosynthetic bacteria do not need solar energy to produce hydrogen gas, there bacteria are more advantageous for producing hydrogen gas than photosynthetic microorganisms.
Among the microorganisms isolated so far, the microorganisms capable of producing hydrogen gas most efficiently is believed to be Enterobacter aerogenes E82005 strain (Tanisho S., et al., Int. J. Hydrogen Energy 12, 623, 1987; Biochim, Biophys, Acta. 973, 1, 1989). However, this strain is a facultative anaerobic bacterium. The strain also grows under anaerobic conditions, but it grows more actively under aerobic conditions. Therefore, when large quantities of hydrogen are produced in a fermenter, it is difficult to maintain aerobic conditions in the fermenter; namely, the use of this strain is not suitable for industrial production of hydrogen gas.